1. Field of the Invention
The present invention generally relates to small propulsion systems for maneuvering spacecraft and, more particularly, is concerned with an arcjet thruster incorporating features which control arc startup and steady state attachment for improved efficiency and optimized performance of the thruster.
2. Description of the Prior Art
As conventionally known, an arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant and from thermal energy to directed kinetic energy by expansion of the heated propellant through a nozzle. For an explanation from an historical perspective of arcjet thruster construction and operation and the problems associated with this type of electrothermal propulsion, attention is directed to the following publications: "Arcjet Thruster for Space Propulsion" by L. E. Wallner and J. Czika, Jr., NASA Tech Note D-2868, June 1965; "The Arc Heated Thermal Jet Engine" by F. G. Penzig, AD 671501, Hollomen Air Force Base, March 1966; and "Physics of Electric Propulsion" by R. G. Jahn, McGraw-Hill Book Company, 1968. Attention is also directed to U.S. Pat. No. 4,548,033 to G. L. Cann.
Although standard constricted arc-heated thrusters have been developed since the early 1960's, using NH.sub.3 or H.sub.2 and producing typically 30% efficiency, these arcjet thrusters have suffered extreme difficulty in starting and have associated therewith very high erosion rates on startup. Heretofore, there has been no way devised to control effectively arc startup and steady state attachment so as to reduce erosion of the thruster block and prolong its life. The arc tends to form as a concentrated radial spike which attaches to the block at a contact point in a subsonic zone thereof, causing at worst complete destruction of the thruster and at best serious pitting and erosion thereof which decreases the number of cycles that can be expected from the thruster. Additionally, the performance and efficiency of these arcjet thrusters are by no means optimized. Recent experimental data have shown that significant gains can be obtained over the performance and efficiency levels reported in the early 1960's. More gains are possible through proper design modifications.
Consequently, a need exists for a fresh approach to arcjet thruster design which will address the problems experienced heretofore with performance and efficiency, and with arc startup and attachment and substantially mitigate their deleterious effects without substituting another, equally stringent, set of problems in their place.